sustainable x occasion wear

I haven't been to a formal event since I was about fifteen years old, and now I'm looking my graduation in the eye. I don't really have anything that I'd feel comfortable wearing- I'll need a dress, shoes, and a bag (and don't know anybody with my size/style to borrow from).

I'd like to make a long-term investment, so I'm trying to focus on:

a) a style that will not look dated next year

b) high quality items (such as leather heels)

c) hopefully something that can grow with me, like a dress with an adjustable corset back

I find it very difficult to justify buying new, but I feel like I'm running in circles trying to find things I like, and then attempting to find the exact same thing in the same colourway, good condition, and a sensible size on vinted... (still listed for £50 of course.) Is it worth just biting the bullet and buying something new so that I KNOW it has the features/colour/sizing that I want (and so I'll hold onto it for longer)? And any advice on fabrics to look for? I keep seeing satin dresses on vinted that are originally £100+ experiencing pilling when they're being sold second-hand!!